In the food industry

Hydrogenation is widely applied to the
processing of vegetable oils and fats. Complete hydrogenation
converts unsaturated fatty acids to
saturated
ones. In practice the process is not usually carried to completion.
Since the original oils usually contain more than one double bond
per molecule (that is, they are poly-unsaturated), the result is
usually described as partially hydrogenated vegetable oil; that is
some, but usually not all, of the double bonds in each molecule
have been reduced. This is done by restricting the amount of
hydrogen (or reducing agent) allowed to react with the fat.

Hydrogenation results in the conversion of liquid
vegetable oils to
solid or semi-solid fats, such as those present in margarine. Changing the degree
of saturation of the fat changes some important physical properties
such as the melting point, which is why liquid oils become
semi-solid. Semi-solid fats are preferred for baking because the
way the fat mixes with flour produces a more desirable texture in
the baked product. Since partially hydrogenated vegetable oils are
cheaper than animal source fats, are available in a wide range of
consistencies, and have other desirable characteristics (e.g.,
increased oxidative stability (longer shelf life)), they are the
predominant fats used in most commercial baked goods. Fat blends
formulated for this purpose are called shortenings.

Health implications

A side effect of incomplete
hydrogenation having implications for human health is the isomerization of the
remaining unsaturated carbon bonds. The cis
configuration of these double bonds
predominates in the unprocessed fats in most edible fat sources,
but incomplete hydrogenation partially converts these molecules to
trans
isomers, which have been implicated in circulatory diseases
including heart
disease (see trans fats).
The catalytic hydrogenation process favors the conversion from cis
to trans bonds because the trans configuration has lower energy
than the natural cis one. At equilibrium, the trans/cis isomer
ratio is about 2:1. Food legislation in the US and codes of
practice in EU has long required labels declaring the fat content
of foods in retail trade, and more recently, have also required
declaration of the trans fat content.

In 2006, New York
City adopted the US's first major municipal ban on most
artificial trans fats in restaurant cooking.

Hydrogenation of coal

History

The earliest hydrogenation is that of platinumcatalyzed addition of hydrogen
to oxygen in the Döbereiner's
lamp, a device commercialized as early as 1823. The French
chemist Paul
Sabatier is considered the father of the hydrogenation process.
In 1897 he discovered that the introduction of a trace of nickel as
a catalyst facilitated the addition of hydrogen to molecules of
gaseous carbon compounds in what is now known as the Sabatier
process. For this work Sabatier won half of the 1912 Nobel
Prize in Chemistry. Wilhelm
Normann was awarded a patent in Germany in 1902 and in Britain
in 1903 for the hydrogenation of liquid oils using hydrogen gas,
which was the beginning of what is now a very large industry world
wide. The commercially very important Haber-Bosch
process (ammonia hydrogenation) was first described in 1905 and
less so Fischer-Tropsch
process (carbon monoxide hydrogenation) in 1922. Another
commercial application is the oxo process
(1938), a hydrogen mediated coupling of aldehydes with alkenes.
Wilkinson's
catalyst was the first homogeneous
catalyst developed in the 1960s and
Noyori asymmetric hydrogenation (1987) one of the first
applications in asymmetric
synthesis. A 2007 review article advocated the use of more
hydrogenations in C-C coupling reactions like the oxo
process.

Metal-free hydrogenation

For all practical purposes,
hydrogenation requires a metal catalyst. Although, there are some
metal-free catalytic systems that are investigated in academic
research. One such system for reduction of ketones consists of tert-butanol
and potassium
tert-butoxide and very high temperatures. The reaction depicted
below describes the hydrogenation of benzophenone:

Another system is based on the phosphine-borane compound (1). It
reversibly accepts dihydrogen at relatively low temperatures to
form the phosphoniumborate 2 which is able to
reduce a simple hindered imine.